The cyanobacteria provide an excellent experimental system for studying the control of gene expression in photoautotrophs. We have been applying the powerful tools of molecular genetics and recombinant DNA technology to the study of the regulation of gene expression in these organisms. We have already cloned genes for several phycobiliproteins, including the genes for phycocyanin and allophycocyanin from Synechococcus 7002. Studies are proposed which would allow the isolation of the genes for all remaining components of the phycobilisomes of this cyanobacterium. Studies to characterize six cloned restriction fragments, which carry coding sequences for phycobiliproteins including phycoerythrin from the chromatically-adapting cyanobacterium Pseudanabaena 7409, are also described. A series of promoter fusions to the lacZ gene of E. coli will be constructed. These constructs will be used to examine phycobiliprotein gene expression in cells subjected to different physiological stresses including nutrient starvation, changes in light intensity, and changes in light wave-length. Through the construction of defined mutations in phycobilisome components, we hope to dissect the controls which co-ordinate the synthesis and assembly of these proteins into functional light-harvesting organelles. These studies should extend significantly our understanding of gene regulation processes in the cyanobacteria.